1. Field of the Invention
The present invention relates generally to a mobile communication network. More particularly, the present invention relates to a mobile communication network for the provision of digital optical transmission through an optical link in the base transceiver system of a digital cellular system (DCS), a mobile telephone network, a personal communication system, a mobile communication system of the next generation (IMT2000), etc.
2. Description of the Related Art
FIG. 1 illustrates a conventional mobile communication system for controlling a plurality of base transceiver systems, which comprises a mobile station (MS) 12; a plurality of base transceiver systems (BTS) 5; a base station controller (BSC) 3 in communication with the BTSs; a mobile switching center (MSC) 2 coupled to the BSC 3; and, a public switching telephone network (PSTN) 1. The mobile station 12 is a terminal unit that allows a subscriber to communicate within the mobile communication networks. The base transceiver systems 5 establish a wireless connection to the mobile station 12 and control the mobile station 12 through the established communication channels. The base station controller 3 controls both wireless and wired connections and couples the existing network to other communication networks. A single base station controller 3 typically employs E1/T1 links for controlling the plurality of BTSs 5. However, the installation cost of the plurality of BTSs is enormous and each BTS only provides a limited cell coverage area. A cell is classified according to its size, i.e., a macro cell (about 5 km-30 km); a micro cell (about 500 m-1 km); and a mega cell using low-orbit satellites (100 km). For example, the reference number 6 represents the cell coverage of a base transceiver system 5.
A plurality of optical repeaters 7 employing a sub-carrier multiplexing (SCM) scheme have been developed to provide services beyond the assigned cell coverage area in the areas where the installation of the base transceiver systems is difficult and the reception of the electromagnetic radiation signals is poor. The optical repeaters 7 are employed to secure a broader cell coverage in the regions where the traffic usage is low. In this prior art system, many remote base transceiver systems (BTSs) includes optical repeaters that are installed within the network with one reference base transceiver system 5 for controlling the optical repeaters 7. In the regions where the installation of a reference BTS 5 is costly and the expected traffic is not so heavy, i.e., skiing resorts, golf courses, streets, remote villages, optical repeaters are used to cover the same regions (i.e., reference number 8 represents the cell coverage of each optical repeater) in the prior art system. To this end, the optical divider 11 is provided in the reference base transceiver system 5 to transmit data to each optical repeater 7 through the optical fibers 10. Thus, the conventional art system has some merit of efficiently reducing the enormous cost of installing the reference base transceiver system 5.
However, the optical divider 11 used in the prior art system has some drawbacks in that the multiple optical fibers 10 has be installed as many as the respective optical repeaters 7. Another drawback is that the optical fibers 10 corresponding to the respective optical repeaters 7 have to be installed around highways, in tunnels and buildings. As each repeater requires a dedicated fiber line, the cost of this type of installation is very high.
Moreover, the conventional mobile communication system is not equipped to prove multimedia service requiring higher speed and capacity, thus causing a problem during an access operation between the optical repeaters and the base transceiver systems.
As the prior art system employs optical repeaters to secure broader coverage beyond the existing cell coverage of the BTSs by means of the optical fibers matching the respective optical repeaters, it has a structural disadvantage in installation around highways or inside buildings. Moreover, if the optical fibers are arranged in parallel, the expenses associated in installing the optical fibers and the dedicated lines will increase dramatically. Furthermore, the distance between the reference base transceiver system and the optical repeaters is limited in the range of 20 Km. Hence, the business sector would have the double burden of installing more optical repeaters as well as the reference base transceiver system.